1. Field of the Invention
This invention relates to a method of and an apparatus for taking a plurality of radiation images at different distances from the object which are suitable to generate a phase contrast image.
2. Description of the Related Art
There has been known a radiation image reproduction system in which an object is exposed to a radiation (X-rays, xcex1-rays, xcex2-rays, electron beams, ultraviolet rays or the like), the radiation passing through the object is detected by the use of, for instance, a stimulable phosphor sheet (to be described later) or a radiation detector panel (to be described later), thereby obtaining a radiation image data representing a radiation image of the object, and a radiation image is reproduced on the basis of the radiation image data after it is variously processed.
When certain kinds of phosphor are exposed to a radiation (X-rays, xcex1-rays, xcex2-rays, electron beams, ultraviolet rays), they store a part of energy of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as xe2x80x9ca stimulable phosphorxe2x80x9d. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as xe2x80x9cstimulated emissionxe2x80x9d. Further, a recording sheet comprising a layer of such a stimulable phosphor is referred to as xe2x80x9ca stimulable phosphor sheetxe2x80x9d. When the stimulable phosphor sheet is used, the stimulable phosphor sheet is exposed to stimulating light after exposed to a radiation passing through an object and the stimulated emission emitted from the stimulable phosphor sheet upon exposure to the stimulating light is photoelectrically read, thereby obtaining image data representing a radiation image of the object. The radiation detector panel comprises a plurality of two-dimensionally arranged detecting elements and the detecting elements generates electric signals proportional to the amount of radiation projected onto the panel. Image data representing a radiation image of the object is obtained on the basis of the electric signals output from the detecting elements.
The radiation image thus obtained represents difference in intensity of the radiation passing through the object. For example, when the object includes a bone and a soft tissue, the radiation passing through the bone is largely attenuated and a very small part of the radiation reaches the detector (e.g., a stimulable phosphor sheet or a radiation detector panel) whereas the radiation passing through the soft tissue is less attenuated and a relatively large part of the radiation reaches the detector. Accordingly, in the case of such an object, the bone is expressed in white and the soft tissue is expressed in black. That is, a radiation image obtained is large in contrast and rich in information.
However, when the object mainly includes only soft tissues like a mammogram, difference in radiation attenuation by tissues is not so large, and accordingly, a radiation image obtained is small in contrast and poor in information.
In order to overcome this problem, there has been proposed a phase contrast imaging in which phase difference of radiation generated when the radiation passes through the object is visualized. The phase contrast imaging is based on the fact that when radiation is projected onto different materials, the phase of the wave of the radiation changes before and after passing through the materials and a phase difference is generated due to difference in propagation in the materials since radiation is an electromagnetic wave like light. When the object is of a soft part, a fine difference in tissues included in the soft part can be more clearly visualized by the phase contrast imaging since the phase difference is larger than the difference in attenuation. The phase contrast imaging is described in detail, for instance, in xe2x80x9cQuantitative aspects of coherent hard X-ray imaging: Talbot images and holographic reconstructionxe2x80x9d by Peter Cloetens, et al., (Proc, SPIE, Vol. 3154(1997), 72-82) (will be referred to as xe2x80x9cpaper 1xe2x80x9d, hereinbelow), and xe2x80x9cHard x-ray phase imaging using simple propagation of a coherent synchrotron radiation beamxe2x80x9d by Peter Cloetens, et al., J. Phys. D:Appl. Phys.32(1999), A145-A151 (will be referred to as xe2x80x9cpaper 2xe2x80x9d, hereinbelow). According to these papers, a phase contrast image can be generated by taking images at a plurality of distances from the object by the use of a two-dimensional sensor (e.g., a radiation detector panel), thereby obtaining a plurality of pieces of image data representing a plurality of radiation images, and carrying out operation based on a predetermined algorithm by the use of the plurality of pieces of image data.
In the phase contrast imaging, it is necessary to obtain a plurality of radiation images by repeating process of taking a radiation image by exposing a two-dimensional sensor fixed to a predetermined position to radiation passing through an object and moving the two-dimensional sensor in parallel to the optical axis of the radiation to another position by the use of a sensor moving means, thereby obtaining a radiation image in each position.
However, the process of stopping the two-dimensional sensor in a plurality of positions and driving the radiation source in each of the positions is not a efficient way of obtaining a plurality of radiation images. It is possible to obtain a plurality of pieces of image data without stopping the two-dimensional sensor when image data representing a radiation image is read in time series. However, since in the two-dimensional sensor like a radiation detector panel, image data is obtained by reading out electric charges which are accumulated in the sensor in response to exposure to radiation, a certain response time is required between exposure of the sensor to the radiation and read out of the image data. Accordingly, an attempt of obtaining a plurality of pieces of image data without stopping the sensor encounters a problem that a radiation image in a certain position can overlap with that in the preceding position, which makes it difficult to obtain a precise radiation image in each position.
In view of the foregoing observations and description, the primary object of the present invention is to provide a method of and apparatus for efficiently and precisely obtaining a plurality of radiation images taken in different positions.
In accordance with a first aspect of the present invention, there is provided a method of obtaining a plurality of radiation images of an object taken in different imaging positions, wherein the improvement comprises the steps of
moving an area sensor, which detects radiation passing through the object, in a direction substantially parallel to the optical axis of the radiation,
detecting that the area sensor reaches one of the imaging positions, and
projecting the radiation onto the area sensor only when the area sensor is in one of the imaging positions.
For example, the area sensor may be a radiation detector panel comprising a plurality of two-dimensionally arranged detecting elements.
The radiation images may be used to generate a phase contrast image.
In accordance with a second aspect of the present invention, there is provided an apparatus for obtaining a plurality of radiation images of an object taken in different imaging positions, wherein the improvement comprises
an area sensor which detects radiation passing through the object,
an area sensor moving means which moves the area sensor in a direction substantially parallel to the optical axis of the radiation,
a switching means which performs switching between a projecting state, where the radiation is projected onto the object, and a non-projecting state, where the radiation is not projected onto the object,
a position sensor which detects that the area sensor reaches one of the imaging positions, and
a switching means control means which causes the switching means to switch from the non-projecting state to the projecting state when the position sensor detects that the area sensor is in one of the imaging positions.
The apparatus may further comprise a phase contrast image generating means which generates a phase contrast image on the basis of the radiation images taken in the different imaging positions.
The switching means may be a means which selectively drives or stops a radiation source which radiates the radiation or a shutter which is disposed on the optical axis of the radiation to selectively interrupt the radiation.
In the method and the apparatus of the present invention, radiation passing through an object is projected onto the area sensor only when the area sensor is one of the imaging positions, which permits to obtain a plurality of radiation images without stopping the area sensor in each imaging position. Further, since the radiation is not projected onto the area sensor when the area sensor is not in the imaging position, the problem that a radiation image in a certain position can overlap with that in the preceding position can be overcome, whereby a plurality of radiation images taken in different positions can be efficiently and precisely obtained.